1. Field of the Invention
The present invention relates to a method and apparatus for controlling a power supply of an electrical discharge machine which supplies machining power to a machining gap between an electrode and a workpiece provided in dielectric.
2. Description of the Background Art
An electrical discharge machine supplies a constant current pulse to a machining gap to melt a workpiece and remove molten material therefrom and to machine the workpiece by the discharge of energy. Generally, the following four conventional power supply circuit arrangements are used to supply the constant current pulse.
One known circuit arrangement for a first power supply apparatus is shown in FIG. 54. This arrangement is, for example, disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 as a "Pulse Generator Used with the Electrical Erosion Machine Tool."
In FIG. 54, the numeral 1 indicates an electrode, 2 denotes a workpiece, 3 designates a control circuit for a switching device 4, 4 represents a switching device, 5 indicates a power supply for supplying a machining current, 6 designates a diode for causing a residual current to flow, 7 represents a current detection resistor, 8a and 8b denote stray inductances of wiring, 9 indicates a comparator, 10 represents an envelope signal generator, and 18 designates a servo device for exercising the servo control of the electrode 1.
The operation of this circuit will now be described. Before a discharge is started, the switching device 4 is conducting and a machining voltage is applied to the machining gap between the electrode 1 and the workpiece 2 by the power supply 5. Upon the start of the discharge, a pulse command 16 corresponding to a machining current waveform to be supplied to the machining gap is output from a control apparatus (not shown in FIG. 54) to the envelope signal generator 10. The pulse command 16 is output by the envelope signal generator 10 as envelope signals 13, 14. FIG. 55 shows the shapes of the envelope signals 13, 14. In the comparator 9, the current flowing in the machining gap is detected by the current detection resistor 7 to obtain a present machining current value 15, thereby comparing the envelope signals 13, 14 with the present machining current value 15 and outputting a control signal 12 to the control circuit 3. The control circuit 3 switches on/off the switching element 4 under the control of the control signal 12 to control the machining current within a predetermined value. Namely, when the present machining current value 15 exceeds the envelope signal 13, the switching device 4 is turned off. Conversely, when the present machining current value 15 falls below the envelope signal 14, the switching device 4 is turned on. The machining current is controlled in the above method.
In this method, the rising speed of the machining current waveform is determined by the current detection resistor 7 and the magnitude of the inductances 8a, 8b of a machining current supply feeder, i.e., the resistor and inductances are used as loads to carry out switching control.
A second conventional circuit arrangement for a power supply apparatus is shown in FIG. 58, which is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. SHO57-33949 as a "Pulse Generation Circuit Controlled for Formation by Intermittent Electrical Discharges". This power supply apparatus has been improved in rising and falling speeds of the machining current, as compared to the first power supply apparatus, in order to ensure faster operation. In FIG. 58, an auxiliary power supply 28, a first switching device 4, a current detector 24, a reactor 22 and a diode 23 constitute a first auxiliary circuit. A power supply 5, the auxiliary power supply 28, the first switching device 4, the current detector 24, the reactor 22, an electrode 1, a workpiece 2 and a second switching device 20 constitute a main circuit.
The operation of this circuit will now be described. In the first auxiliary circuit, the switching device 4 is driven by a control circuit 27 under the control of the detection signal of the current detector 24. The control circuit 27 carries out the switching control of the switching device 4 so as to render the current flowing in the current detector 24 constant. In this case, the reactor 22 inserted in the circuit allows the current flowing in the first auxiliary circuit to be kept constant.
This second power supply apparatus is fitted with a second switching device 20 exclusively employed to switch the discharge pulse on/off. When the discharge pulse is off, a current within a predetermined range flows in the first auxiliary circuit on a steady-state basis, and as soon as the discharge is started, the machining current is supplied from the first auxiliary circuit. This enables the current to rise extremely fast. The current during the discharge flows in the main circuit which consists of the power supply 5, the auxiliary power supply 28, the first switching device 4, the current detector 24, the reactor 22, the electrode 1, the workpiece 2 and the second switching device 20. When the discharge has ended, the current which had been flowing in the reactor 22 of the main circuit flows to the second diode 23 in the first auxiliary circuit, thereby intercepting the current of the machining gap rapidly.
A first diode 25 is provided to raise power supply efficiency by forming a second auxiliary circuit and causing the current flowing in the reactor 22 to return to the power supply 5 when the first switching device 4 and the second switching device 20 are both switched off. The second auxiliary circuit is constituted by the first diode 25, the current detector 24, the reactor 22, the second diode 23 and the main power supply 5. FIG. 59 shows a machining current waveform generated by the second power supply apparatus.
Also, there is a third conventional circuit arrangement for a power supply apparatus shown in FIG. 60, which is disclosed, for example, in Japanese Laid-Open Patent Publication No. HEI2-34732 as a "Control Method for the Electrical Discharge Machining Power Supply." In FIG. 60, 30a to 30e indicate drive devices which cause switching devices 32a to 32e to conduct and which constitute a logic circuit 35. 33a to 33e represent limiting resistors which control a machining current and which have different values individually. Between an electrode 1 and a workpiece 2 is a detector 36 for detecting a discharge start. This detector 36 transmits a discharge detection signal 37 to the logic circuit 35. The logic circuit 35 selects the switching devices 32a to 32e to be driven under the control of the output signal of an oscillator 34 and the discharge detection signal 37.
The operation of this circuit will now be described. In the circuit, a power supply 5 is provided for supplying a current and a parallel connection of circuits, each comprising series connections of the switching devices 32a to 32e and the current limiting resistors 33a to 33e, is connected in series with the power supply 5. The resistance values of the current limiting resistors 33a to 33e different from each other are designed to be a power of two, i.e., once, twice, four times, etc. When a rectangular wave having a constant current value and a duration t.sub.p as shown in FIG. 61 is to be supplied, some of the switching devices 32 are switched on by their corresponding drive circuit 30 to cause current to flow through the corresponding current limiting resistors 33. When the discharge is started, a machining current is supplied to the machining gap through selected resistors 33. A difference voltage between the output voltage of the main power supply 5 and the discharge voltage generated at the machining gap between the electrode 1 and the workpiece 2 is applied to each current limiting resistor, thereby determining the current flowing in the current limiting resistor. Since the discharge voltage is generally a constant value, the machining current is determined uniquely by the selection of the current limiting resistors.
Further, as shown in FIG. 62, the rising speed of a current waveform can be controlled. By switching the switching devices 32 on/off continuously after the discharge current has risen up to a point indicated by 48 in FIG. 62, the current can be further increased but can be raised with its slope further reduced. Such intentional control of the discharge current waveform is often exercised to provide finer control of the machining operation.
Finally, there is a circuit arrangement for a fourth conventional power supply apparatus shown in FIG. 63, which is disclosed, for example, in the specification of U.S. Pat. No. 4,306,135. In this drawing, 49 indicates a fixed current limiting resistor, 50 denotes a semiconductor amplifier such as an FET, 51 designates a switching device for switching the semiconductor amplifier 50 on/off to turn a discharge pulse on/off, 52 represents a digital signal which specifies the current waveform shape of the discharge pulse, 53 denotes a digital-to-analog converter which converts said digital signal into an analog signal, 54 indicates an amplifier for driving the amplifier 50, and 55 represents a limiting resistor for the amplifier 54.
The operation of this circuit will now be described. For the ON/OFF timing of the discharge pulse, a signal is output from the oscillator 21 to drive the switching device 51. The current supplied to the machining gap between the electrode 1 and the workpiece 2 after the discharge has occurred is determined by the resistance values of the fixed resistor 49 and semiconductor amplifier 50. When, for example, an FET is used as the semiconductor amplifier 50, it can be operated as a variable resistor.
The characteristic of the FET is as indicated in FIG. 64. When VGS is determined optionally, ID is kept constant if VDS varies slightly. Namely, the FET features that the machining current is controlled to be kept constant independently of slight variation of the power supply voltage 5. For this reason, the current during discharge is stable and so-called pulse interruption, i.e., discharge stopping halfway into the pulse, is unlikely to occur, thereby providing extremely stable machining.
Also, changing a signal to the gate G of the FET 50 within a single pulse provides an optional waveform and offers a constant-current characteristic to a command value G, ensuring especially stable machining.
The conventional electrical discharge machining power supply constructed as described above has the following disadvantages.
Namely, since the "Pulse Generator Used the Electrical Erosion Machine Tool" disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 attempts to essentially control the machining current value within the specified range in switching control, the machining current waveform 47 has a ripple, as shown in FIG. 55. This ripple is generally several amperes in width. Examples of machining current pulses generated under various conditions are shown in FIGS. 56 and 57. FIG. 56 shows a large machining current value setting, i.e., current setting for so-called roughing. In a current waveform 47b of this example, the ripple width (which is approximate to a gap between command values 13 and 14) is small relative to a peak value 13 of the machining current command value and therefore does not cause any particular fault to machining. However, when the command value of the current peak value is reduced, as shown in FIG. 57, the lower limit value 14 of the command value no longer is significant and the waveform originally desired to be rectangular becomes triangular as indicated by 47c. As a result, the pulse cannot be sustained for a desired period of time and becomes intermittent. This waveform is not capable of providing a desired machining result.
Because of the ripple in the current waveform, the current waveform to be controlled by switching control is inappropriate for the control of a microcurrent waveform as in finishing and cannot achieve desired machining.
Also, the "Pulse Generation Circuit Controlled for Formation by Intermittent Electrical Discharges" disclosed in Japanese Laid-Open Utility Model Publication No. SHO57-33949 is designed to solve the disadvantages of the technique disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 to some extent. Namely, the reactor 22 inserted in the circuit of FIG. 58 keeps the current constant more easily and the ripple width of the current waveform can be considerably smaller than that in the technique of the circuit in FIG. 54. Generally, the insertion of a reactor allows the current to be kept constant more easily but has a disadvantage that rising and falling speeds cannot be provided. In the circuit of FIG. 58, however, the first auxiliary circuit is employed to secure the peak current value in advance, and upon the start of the discharge pulse, the separate second switching device 20 is used to cause the discharge circuit to conduct or not to conduct, thereby improving the rising and falling speeds. However, the auxiliary power supply 28 is required for this purpose.
This auxiliary power supply 28 is likely to be considerably larger in output capacity than the power supply 5 serving as a main power supply because it may be smaller in output voltage but its output current must be substantially equal to the machining current. Namely, another disadvantage of this technique is a difficulty in providing the circuit at low cost.
Further, the technique disclosed in Japanese Laid-Open Utility Model Publication No. SHO57-33949 has a disadvantage that, unlike the technique disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928, it is difficult to provide an optional discharge pulse waveform concurrently with the control of the rising and falling speeds thereof and only the rectangular wave as shown in FIG. 59 may be offered.
Also, the technique disclosed in the "Control Method for the Electrical Discharge Machining Power Supply" in Japanese Laid-Open Patent Publication No. HEI2-34732 is built to solve the disadvantages in the techniques disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 and Japanese Laid-Open Utility Model Publication No. SHO57-33949.
The technique disclosed in Japanese Laid-Open Patent Publication No. HEI2-34732 employs the constant-voltage power supply and the resistor inserted therein to control the machining current value, without the discharge machining current being controlled by switching control. Hence, the discharge current waveform provided has almost no current ripple as shown in FIG. 61 and switching on/off the resistors 33a to d in the circuit at high speed allows the rising speed as indicated by 48 and the current waveform shape to be set optionally as shown in FIG. 62.
However, the disadvantage of this technique is that the current is not controlled directly but is controlled according to the resistance value which limits the current, whereby the discharge current value varies according to the output voltage of the power supply 5. In other words, if a given current value has been set, the same machining status cannot be provided when the power supply voltage varies.
Further, it is known that the discharge gap between the electrode 1 and the workpiece 2 physically acts as a constant-voltage load of approximately 25 V. For this reason, the difference voltage between the output voltage of the power supply 5 and the 25 V voltage drop of the discharge gap is mostly applied to the current limiting resistors 33 and is consumed as thermal energy. Namely, as the power supply of the electrical discharge machine, this technique cannot avoid a reduction in power supply efficiency as compared to the techniques disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 and Japanese Laid-Open Utility Model Publication No. SHO57-33949. This hinders the downsizing of the power supply apparatus and also makes it difficult to implement the same functions at low costs. As described above, the apparatus in information 3 had the disadvantages that the machining current is not kept constant easily, that the power supply efficiency is poor, and that this poor power supply efficiency resulted in large size and high price of the apparatus.
Further, in the specification of U.S. Pat. No. 4,306,135, some disadvantages of the technique disclosed in Japanese Laid-Open Patent Publication No. HEI2-34732 have been solved.
Namely, the technique disclosed in the specification of U.S. Pat. No. 4,306,135 uses a semiconductor amplifier instead of the plurality of current limiting resistors in the technique disclosed in Japanese Laid-Open Patent Publication No. HEI2-34732. Since the FET is employed as the semiconductor amplifier in this conventional art, the constant-current characteristic as shown in FIG. 64 is provided. That is, a constant current can be maintained and controlled relative to the variation in output voltage the power supply 5, and besides, the constant-current control can also be exercised during the continuation of the discharge pulse, whereby extremely stable machining can be achieved. In the sense that the in-pulse current can be rendered constant, more stable machining can be attained as compared to the switching power supplies in the techniques as disclosed in Japanese Laid-Open Patent Publication No. SHO62-27928 and Japanese Laid-Open Utility Model Publication No. SHO57-33949. Further, since the resistance value at the time of discharge start is extremely small as compared to Japanese Laid-Open Patent Publication No. HEI2-34732, the discharge current can be raised faster.
However, the difference voltage between the output voltage of the power supply 5 and the machining gap voltage is all applied to the semiconductor amplifier 50. Namely, the thermal energy to be consumed by the semiconductor amplifier 50 is large. As compared to ordinary electrical parts, the semiconductor is easily affected particularly by heat and has importance in heat-dissipation design. However, the technique disclosed in the specification of U.S. Pat. No. 4,306,135 generates much heat because it not only uses the semiconductor as the switching device but also employs it as the variable resistor in an active range, i.e., this technique does not allow a large current to flow and it is very difficult to design a circuit which can achieve electrical discharge roughing requiring the current peak value of tens of amperes or higher.
Namely, the technique disclosed in the specification of U.S. Pat. No. 4,306,135 has a disadvantage that a large current adequate for roughing cannot be controlled.
It is accordingly an object of the present invention to overcome these disadvantages by providing a power supply of an electrical discharge machine which has a small ripple in a machining current pulse, allows a microcurrent to be formed easily in finishing, and allows a power supply apparatus to be small in size and low in costs because of its extremely high power supply efficiency.